Integral compressor-expander

ABSTRACT

An integral compressor-expander assembly, including a cryogenic expander positioned in an overhung configuration on a central shaft; a multi-stage centrifugal compressor supported on the central shaft between at least two bearings; and a device coupled to the central shaft and configured to either supply rotational power to the central shaft or generate power from rotation of the central shaft, depending upon a current operational mode of the multi-stage compressor.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/295,633, which was filed Jan. 15, 2010 and to U.S.Provisional Patent Application Ser. No. 61/303,270, which was filed Feb.10, 2010. These priority applications are hereby incorporated byreference in their entirety into the present application, to the extentthat they are not inconsistent with the present application.

BACKGROUND

This disclosure relates in general to an integral compressor-expanderassembly used in refrigeration applications, particularly, in theliquefaction of natural gas. The term “integral” is generally defined tomean that the compressor and expander are mounted on a single, commonshaft. In such refrigeration applications, cold gas expanders are usedto produce low temperatures via pressure reduction of a flowing gas.Typically, mechanical energy is recovered from the expander to serve auseful purpose. The mechanical energy recovered from the expander can beprovided to an electric generator, or to a compressor adapted tocompress a gas stream. Recovering the energy directly to compression isusually the most efficient and cost effective means as it eliminatescostly power generation equipment and associated energy losses.

Typically, when energy is recovered to compression, the configuration ofthe assembly is a single expander with radial inflow and axial outflowoverhung on one end of a central shaft, and a single-stage compressorwith axial inflow and radial outflow overhung on the other end of theshaft. In this configuration, the compression duty is constrained toprecisely match the expansion duty to keep the assembly in power balancewith no external driver or load. In addition, the pressure rise in thecompressor is constrained to the amount that can be achieved by a singleimpeller.

When the expansion duty exceeds the compression duty or vice versa, anintegrally geared arrangement may be used. With the integrally gearedarrangement, multiple compressors with axial inlets and radialdischarges may be driven by a single gear on multiple shafts, with theexpander driving this same gear from another shaft. The gear may also becoupled to an external driver to provide additional power in the eventthe compression duty exceeds the expansion duty. The integral geararrangement requires several bearings and seals, making its designcomplicated and leading to lower reliability and higher frequencies ofmachine downtime.

Therefore, there is a need to facilitate efficient energy recovery whileachieving higher compression ratio in a single, reliable assembly.

SUMMARY

Embodiments of the disclosure may provide a compressor-expanderassembly. The assembly may include a cryogenic expander positioned in anoverhung configuration on a central shaft and a multi-stage centrifugalcompressor supported on the central shaft between at least two bearings.The assembly may further include a device coupled to the central shaftand configured to either supply rotational power to the central shaft orgenerate power from rotation of the central shaft, depending upon acurrent operational mode of the multi-stage compressor.

Embodiments of the disclosure may also provide a method includingexpanding a fluid in a cryogenic expander, wherein the cryogenicexpander is coupled to a central shaft to which a multi-stagecentrifugal compressor and a device are also coupled. The method mayfurther include rotating the cryogenic expander and creating a poweroutput therefrom. In at least one embodiment, the device and thecryogenic expander drive the compressor if the power from the cryogenicexpander is less than that required to drive the compressor. In anotherembodiment, the cryogenic expander may drive the device and thecompressor if the power from the cryogenic expander is more than thatrequired to drive the compressor. In yet another embodiment, thecryogenic expander may drive only the compressor if the power from thecryogenic expander is not more or less than required to drive thecompressor.

Embodiments of the disclosure may also provide an apparatus including acryogenic expander. During operation of the apparatus, the cryogenicexpander may receive and cool a through flow of fluid entering theexpander at ambient temperature or below. The apparatus may also includea compressor and a shaft operably coupling the expander and thecompressor, where the shaft has a first end portion, a second endportion, and a longitudinal portion disposed between the expander andthe compressor. The apparatus may also include a bearing rotationallysupporting the longitudinal portion of the shaft and a casing enclosingthe expander, the compressor, the first end portion of the shaft and thelongitudinal portion of the shaft, with the second end portion of theshaft extending outwardly through the casing and adapted to beoperatively coupled to a device operative to supply rotational power tothe shaft, or to receive rotational power from the shaft, duringoperation of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying Figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic view of an exemplary integral compressor-expander,in accordance with the disclosure.

FIG. 2 is a schematic view of the integral compressor-expander in anexemplary embodiment where the expander and compressor are in separatehousings joined together.

FIG. 3 is a schematic view of the integral compressor-expander in anexemplary configuration where a device is coupled to the central shaft.

FIG. 4 is a flow chart of an exemplary method of driving a compressorusing the integral compressor-expander.

DETAILED DESCRIPTION

It is to be understood that the following disclosure describes severalexemplary embodiments for implementing different features, structures,or functions of the invention. Exemplary embodiments of components,arrangements, and configurations are described below to simplify thepresent disclosure, however, these exemplary embodiments are providedmerely as examples and are not intended to limit the scope of theinvention. Additionally, the present disclosure may repeat referencenumerals and/or letters in the various exemplary embodiments and acrossthe Figures provided herein. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various exemplary embodiments and/or configurationsdiscussed in the various Figures. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact.Finally, the exemplary embodiments presented below may be combined inany combination of ways, i.e., any element from one exemplary embodimentmay be used in any other exemplary embodiment, without departing fromthe scope of the disclosure.

Additionally, certain terms are used throughout the followingdescription and claims to refer to particular components. As one skilledin the art will appreciate, various entities may refer to the samecomponent by different names, and as such, the naming convention for theelements described herein is not intended to limit the scope of theinvention, unless otherwise specifically defined herein. Further, thenaming convention used herein is not intended to distinguish betweencomponents that differ in name but not function. Further, in thefollowing discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to.” All numericalvalues in this disclosure may be exact or approximate values unlessotherwise specifically stated. Accordingly, various embodiments of thedisclosure may deviate from the numbers, values, and ranges disclosedherein without departing from the intended scope. Furthermore, as it isused in the claims or specification, the term “or” is intended toencompass both exclusive and inclusive cases, i.e., “A or B” is intendedto be synonymous with “at least one of A and B,” unless otherwiseexpressly specified herein.

In an exemplary embodiment, as illustrated in FIG. 1, an integralcompressor-expander 10 that can be utilized for a multitude offunctions, one of which may be to liquefy natural gas, is shown. Theintegral compressor-expander 10 may include a pressurized casing 12enclosing a radial inflow/axial outflow cryogenic expander 14representatively overhung on a first end portion of a central shaft 16and a radial inflow/radial outflow multi-stage or multi-wheelcentrifugal compressor assembly 18 axially-offset from the cryogenicexpander 14 along a longitudinal portion of the central shaft 16. Inanother embodiment that expander 14 and the compressor 18 may be inseparate casings that are coupled or otherwise attached together. Thecentrifugal compressor assembly 18 is representatively shown in anorientation in which a high pressure side of the compressor assembly 18is farther away from the cryogenic expander 14 than a low pressure sideof the compressor assembly 18. Further, although a pressurized casingwill be generally described herein, the inventors contemplate that anon-pressurized casing could be used to implement embodiments of thepresent disclosure without departing from the scope thereof.

The orientation and/or configuration of the centrifugal compressorassembly 18 may be varied to suit a particular processing requirement,space, or other parameter that may conventionally be used to select acompressor unit. For example, one exemplary configuration of theassembly 18 is where the high pressure side of the compressor assembly18 is adjacent the cryogenic expander 14. In other embodiments, the highpressure side may be positioned distant the expander 14. Additionally,the compressor could be in any number of other configurations, such asback to back, double flow, or compound compressor configurations withoutdeparting from the inventors' intended scope of the disclosure.

At least two radial bearings 20 a and 20 b and at least one thrustbearing 22 may be located within the pressurized casing 12. Moreover,numerous other internal seals can be implemented inside the casing 12and configured to constrain internal leakages. The specificconfiguration and type of seals would depend on the application, buttheir various forms of implementation, although not specificallyillustrated herein, do not depart from the scope of the presentdisclosure.

In at least one embodiment, the thrust bearing 22 can be located inboard(i.e., to the left of) from the radial bearing 20 b, as illustrated.However, the thrust bearing 22 can also be disposed outboard (i.e., tothe right of) from the radial bearing 20 b. Having the thrust bearing 22disposed outboard of the radial bearing 20 b may allow the thrustbearing 22 to include a larger active area, thereby increasing itsefficiency. In other exemplary embodiments, the radial bearing 20 b andthrust bearing 22 can be located externally from the casing 12. Locatingthe radial bearing 20 b and thrust bearing 22 outside of the casing 12may prove advantageous in that the bearings 20 b, 22 may be moreaccessible for assembly or maintenance purposes, and further, if thesecomponents are positioned outside of a pressurized casing, some of thechallenges associated with operating bearings and thrust bearings may beavoided.

In at least one embodiment, the radial bearings 20 a and 20 b may bemagnetic bearings, coupled with at least one catcher or coast downbearing (not shown) that may be configured to temporarily support therotating shaft in the event of a magnetic bearing failure. As known inthe art, magnetic bearings may function under pressure and, therefore,may generally meet minimal sealing standards for the processes disclosedherein. Also, magnetic bearings generally enjoy a wider range oftemperature independence, which may prove advantageous in embodiments ofthe disclosure where temperatures are routinely below freezing, and intocryogenic temperature ranges. Moreover, magnetic bearings may be exposeddirectly to non-corrosive process fluids during operation, and yetcontinue to function properly.

Other kinds of bearings commonly used in turbomachinery, can also beused instead of, or in addition to, magnetic bearings. For example, inat least one embodiment, the radial bearings 20 a and 20 b may includelubricated oil bearings. Depending on the structural location of thebearings 20 a and 20 b (e.g., within out outside the casing 12),lubricated oil bearings can either have a pressurized lube-oil drain oran atmospheric lube oil drain with a pressurized supply. At least oneadvantage to using lubricated oil bearings may be that other parts ofthe integral compressor-expander 10 may also use lube-oil, whetherreceived under pressure or at atmospheric pressures, such as a gear boxor a motor generator. Thus, there would be no additional complexity tothe implementation of a lube oil system to support bearings. However,the inventors recognize that the oil used in the lube oil system wouldlikely need to be maintained at an acceptable temperature forlubrication, i.e., heated to maintain the appropriate viscosity, whichis within the scope of the present disclosure.

As illustrated, the radial bearing 20 a, which can be located betweenthe cryogenic expander 14 and the centrifugal compressor assembly 18,may be exposed to fluid flowing through the cryogenic expander 14 and/orthe centrifugal compressor assembly 18. In an exemplary embodiment, theradial bearing 20 a may be an oil lubricated bearing, wherein oil issupplied to the radial bearing 20 a at an elevated pressure such thatthe pressure in a drain line of the radial bearing 20 a is coincidentwith a normal operating pressure between the cryogenic expander 14 andcentrifugal compressor assembly 18, so that additional seals at thebearing location are not required. However, the implementation of otherseals, such as labyrinth seals or other passive seals, may be includedin these regions without departing from the scope of the disclosure. Inan alternative embodiment, at least one bearing and/or seal may bepositioned radially outward of the expander outlet, thus removing theoverhung configuration. However, in this embodiment, the bearing and/orseal would likely require special construction/materials to be able toeffectively operate in the cryogenic temperature range that is likely atthe output of the expander 14.

A balance piston 23 may be located within the pressurized casing 12. Thesize and location of the balance piston 23 depend on axial forces thatare developed during operation of the integral compressor-expander 10.In one or more embodiments, the balance piston 23 may be passive, or maybe controlled using control elements (not shown) in a manner known tothose of skill in the art. However, in other embodiments, the balancepiston 23 may be controlled using known techniques, such as pressurizedgas or oils. At least one seal 24 may be disposed on the central shaft16 adjacent an opening 25 in the pressurized casing 12. In at least oneembodiment, the second end portion of the central shaft 16 that is notdisposed within the pressurized casing 12 may extend through the opening25. In operation, the seal 24 may be configured to substantially preventleakage of fluids, such as process gas, outward through the opening 25of the pressurized casing 12.

In at least one embodiment, the seal 24 may be a dry gas seal, whichgenerally has the least amount of leakage in similar applications.Nitrogen may be used as the feed gas to the dry gas seal, but processgas could also be used if it were conditioned to the proper pressuresand conditions. In other embodiments, the seal 24 may include at leastone labyrinth seal. As known in the art, labyrinth seals are fairlyinexpensive and predictably reliable. However, other kinds of passiveseals (i.e., seals that do not require an external input but rely on thepressure differential to function) may also be used. For example, one ormore brush seals may be used as the seal 24.

As can be appreciated, several different configurations can exist in howthe radial bearings 20 a and 20 b, the thrust bearing 22, and the seal24 are disposed in the integral compressor-expander 10 system. Dependingon the application, for example, the seal 24 can operate either inboardor outboard of the radial bearing 20 b and thrust bearing 22. Thus, inat least one embodiment, both the radial bearing 20 b and thrust bearing22 may be located externally from the casing 12, as described above,while the seal 24 functions to prevent fluid leakage through the opening25. In other exemplary embodiments, only one of either the radialbearing 20 b or thrust bearing 22 may be located externally from thecasing 12, having the seal 24 interposed between the two.

During exemplary operation, a pressurized fluid 26 may enter thecryogenic expander 14 radially, expand therein, and exit axially. Inanother exemplary embodiment, the system may be configured such that thefluid may initially enter the centrifugal compressor assembly in aradial direction, and after the fluid is compressed, the fluid exitsfrom the compressor assembly in a radial direction. Thus, it is apparentthat the present disclosure provides for both radial and axial fluidinput, as well as radial and axial fluid output. However, in the primaryexemplary embodiment being discussed in this disclosure and shown in theFigures, the input to the compressor is radial and the exit is axial,although embodiments of the disclosure are clearly not limited to thisparticular configuration, as both radial and axial compressorinputs/outputs are contemplated.

Regardless of the particular configuration of inputs/outputs, inoperation the expansion of the pressurized fluid 26 imparts energy tothe cryogenic expander 14 and causes the cryogenic expander 14 torotate. Rotation of the cryogenic expander 14 may, in turn, cause thecentral shaft 16 to rotate, thereby causing the impellers of thecentrifugal compressor assembly 18 to rotate. In one or moreembodiments, fluid 28 may initially enter the centrifugal compressorassembly 18 radially and is subsequently directed to flow axially intothe rotating impellers of the centrifugal assembly 18, where the fluid28 is compressed by the rotating impellers of the centrifugal compressorassembly 18. Compressed fluid 28 may exit radially from the centrifugalcompressor assembly 18.

For the sake of clarity, inlet conditions of fluids 26 and 28 aredenoted as 26 a and 28 a in the Figures, respectively, and exitconditions of fluids 26 and 28 are denoted as 26 b and 28 b in theFigures, respectively. Representative ranges in inlet and exittemperature and pressure of the fluids 26 and 28 are provided in Table 1below. However, Applicants note that each of the temperatures noted inthe table are approximate (about the indicated temperature) and may vary(in range) by ±5%, 10%, 15%, 20%, or 30%. Therefore, the input streamtemperature (26 a), for example, may be as cold as −195° C. where thelisted temperature is −150° C. Similarly, the input temperature rangefor stream 26 a may be about −195° at the coldest (30% colder than 150)or about 65° C. at the warmest (30% warmer than 50° C.).

TABLE 1 Approximate Approximate pressure temperature Min Max Min Max 26a2 bara 165 bara −150° C.  50° C. 26b 1 bara  50 bara −170° C.  15° C.28a 1 bara  50 bara −150° C. 200° C. 28b 2 bara 165 bara −130° C. 260°C.

As briefly described above, in other exemplary embodiments, the generaldisposition or configuration of the compressor components of thecompressor assembly 18 can be reversed without departing from the scopeof the disclosure. For example, the compressive direction of theimpellers may allow the fluid 28 to enter the compressor assembly 18 andmove axially from right to left, with respect to the Figures. In atleast one embodiment, the balance piston 23 can also be moved to theother side of the compressor assembly 18 to compensate for the reversalof thrusts attained through the varying embodiments and/orconfigurations of the compressor assembly 18. Furthermore, the number ofrotating impellers, or “stages,” can be increased in applications wherehigher compression ratios can be achieved.

Operation of the cryogenic expander 14 and centrifugal compressorassembly 18 may give rise to axial forces along the central shaft 16.The axial forces may be supported by the thrust bearing 22 and thebalance piston 23. Radial forces, which are potentially generated by therotating shaft 16, and rotor weight may be supported by the radialbearings 20 a and 20 b. Additional conduits (not shown) may be presentto provide sealing fluids, vents and valves as needed for operation ofthe bearings 20 a, 20 b and 22, balance piston 23 and seal 24.

Since the expander 14 may not always be in thrust balance with thecompressor assembly 18, the balance piston 23 can, in at least oneembodiment, be replaced (or supplemented) with an active thrustbalancing system (not shown). In at least one embodiment, the activethrust balancing system may include a system adapted to control thepressure of a cavity defined within the casing 12 through the use of anexternal valve (not shown). The valve may be configured to regulate thebleeding of the pressure within the cavity back to a lower,predetermined pressure. In one or more embodiments, the cavity may belocated behind the expander 14, and be fluidly coupled via the valve toa location in front of the expander 14 where the pressure issubstantially lower. The balance diameter of the cavity located behindthe expander 14 could be adapted to provide a thrust force at normaloperating conditions in order to counter the thrust that may begenerated at the opposing sealed end, where the seal 24 is located. Inat least one embodiment, the resulting thrust force derived from thecavity may be configured to generate a net zero thrust on the expander14 at normal operating conditions. In the event the valve fails andthere is no balance piston 23 backup, the valve may be designed to failin an open condition, thereby giving the cavity located behind theexpander 14 a pressure equal or substantially equally to that of theexpander outlet 14. Consequently, in the event of valve failure, the netthrust on the shaft 16 would be equal to the thrust due to the sealedend.

In an exemplary embodiment, the pressurized casing 12 may be fabricatedas one piece to house the components of the integral compressor-expander10. In other exemplary embodiments, as illustrated in FIG. 2, thepressurized casing 12 may be composed of two pieces, a cryogenicexpander housing 34 and a compressor housing 36, that are directlycoupled together in a contiguous relationship along an interface 38. Inat least one embodiment, the cryogenic expander housing 34 andcompressor housing 36 can be coupled together using a series of bolts(not shown), but the two housings 34, 36 may also be coupled by weldingor other known methods for securing casings into a unitary body. Asillustrated in FIG. 2, the bearings 20 a, 20 b and 22 and seal 24 can behoused within the compressor housing 36. In other embodiments,additional bearings and/or seals may also be located at the opening 25.

The compression ratio of the centrifugal compressor assembly 18necessary to achieve a target pressure and temperature of the fluid 28is not necessarily constrained by the power output of the cryogenicexpander 14. As illustrated in FIG. 3, a shaft coupling 56 may be usedto couple a device 58, which is supported on a shaft 16 a, to the secondend portion of the central shaft 16. In at least one embodiment, shaft16 a may be a continuation of the central shaft 16, or a separateindependent shaft. The shaft coupling 56 may be a rigid coupling or aflexible coupling, depending on the application. A speed increasing ordecreasing gear (not shown) may also be coupled between the device 58and the integral compressor-expander 10.

In an exemplary embodiment, the device 58 may be adapted to supplyrotational power to the shaft 16, receive rotational power from theshaft 16, or supply rotational power to and receive rotational powerfrom the shaft 16, depending upon a current operational mode of thecompressor assembly 18. In exemplary embodiments where the device 58 isconfigured to receive rotational power from the shaft 16, the device 58may include a generator and/or compressor. In exemplary embodimentswherein the device 58 supplies rotational power to the shaft 16, thedevice 58 may include a motor or turbine. However, there may also beexemplary embodiments where the device 58 includes a combination of amotor and a generator, wherein the device 58 can be configured to supplyrotational power to the shaft 16 in one operating mode and receive therotational power from the shaft 16 in another operating mode. In atleast one embodiment of the disclosure, the device 58 may include a highspeed high frequency motor, as is often times used in the art to drivehigh speed compression equipment where a turbine is not practical orotherwise desired.

In operation, when the power from the cryogenic expander 14 is less thanthat required to drive the compressor assembly 18, the device 58 can beadapted to supply additional rotational power to the shaft 16. In thisconfiguration, the combination of the device 58 and cryogenic expander14 may cooperatively drive the compressor assembly 18. If the input ofrotational power from the cryogenic expander 14 is more than thatrequired to drive the compressor assembly 18, the device 58 may receiverotational power from the shaft 16. In this configuration, the device 58and the compressor assembly 18 can be driven by the cryogenic expander14. If the power from the cryogenic expander 14 is not more or less thanthat required to drive the compressor assembly 18, the cryogenicexpander 14 drives only the compressor assembly 18. In configurationswhere the device 58 receives power from the shaft 16, the device 58 maybe configured, for example, to either generate electricity (as agenerator) or to further process as fluid (as a compressor). In eitherembodiment, the device 58 may be configured to capture excess powergenerated by the expander 14 and provide useful work product therefrom.

In another embodiment, the operation of the expander 14, compressor 18,and the additional device 58 may be controlled by an electroniccontroller. For example, a controller (not shown) may be configured toreceive inputs representative of the power status of each of thecomponents and generate control signals responsive thereto. As such, ina situation where the expander 14 is providing excess power to thecompressor 18, then the controller may be configured to activate thedevice 58 (an electric motor/generator) to receive and convert theexcess power into electricity that may then be used to run otherequipment or transmitted back to the electrical supply grid so that acost credit may be received. Further, in the situation where theexpander 14 is not providing enough power to the compressor 18 togenerate the desired compression, then the controller may be configuredto activate the device 58 (an electric motor/generator) to provideadditional rotational power to the shaft 16 to supplement the rotationalpower provided by the expander 14. Thus, the controller may beconfigured to control the operation of each of the components of theentire system based upon sensed inputs and a predetermined algorithmthat determines what state each of the components should be operating inunder the current circumstance/sensed inputs.

In an exemplary embodiment, as illustrated in FIG. 4, a method ofdriving a compressor is generally referred to by the reference numeral70 and includes expanding a fluid in a cryogenic expander, coupled to acentral shaft to which a multi-stage centrifugal compressor and a deviceare also coupled, to rotate the cryogenic expander and create a poweroutput therefrom as indicated in block 72. If the power from theexpander is less than that required to drive the compressor, the deviceand the cryogenic expander drive the compressor as indicated in block74. The device and the compressor are driven by the cryogenic expanderif the power from the cryogenic expander is more than that required todrive the compressor as indicated in block 76. The cryogenic expanderdrives only the compressor if the power from the cryogenic expander isnot more or less than that required to drive the compressor as indicatedin block 78.

Although the present disclosure has representatively describedembodiments relating to the liquefaction of natural gas, it isunderstood that the apparatus, systems and methods described hereincould be applied to other environments without departing from the scopeof the disclosure. For example, according to another exemplaryembodiment, rotating machinery used in industrial refrigeration may beconfigured to use embodiments of the integral compressor-expandersystems as described above.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A compressor-expander assembly, comprising: a cryogenic expanderpositioned in an overhung configuration on a central shaft; amulti-stage centrifugal compressor supported on the central shaftbetween at least twobearings; a balance piston positioned along thecentral shaft between an outlet of the multistage centrifugal compressorand one of the at least two bearings; and a thrust bearing positionedalong the central shaft.
 2. The compressor-expander assembly of claim 1,further comprising: an electric motor generator combination coupled tothe central shaft and configured to supply rotational power to thecentral shaft or generate power from rotation of the central shaft,depending upon a current operational mode of the multi-stage compressor.3. The compressor-expander assembly of claim 1, further comprising arotating machinery device coupled to the central shaft, wherein therotating machinery device is configured to either provide rotationalpower to or receive rotational power from the central shaft.
 4. Thecompressor-expander assembly of claim 1, wherein the cryogenic expanderis a radial input axial output expander.
 5. The compressor-expanderassembly of claim 1, wherein the bearings comprise at least one ofradial magnetic bearings and lubricated oil bearings.
 6. Thecompressor-expander assembly of claim 2, wherein: the cryogenic expanderand the multi-stage centrifugal compressor are contained in a singlecasing; and the central shaft extends from the single casing and iscoupled to the electric motor generator combination.
 7. Thecompressor-expander assembly of claim 1, wherein the cryogenic expanderis contained in a first casing and the multi-stage centrifugalcompressor is contained in a second casing, the first and second casingsbeing coupled together.
 8. (canceled)
 9. The compressor-expanderassembly of claim 1, wherein the cryogenic expander is configured toreceive an input fluid stream that is at a temperature of between about50° C. and about −150° C.
 10. The compressor-expander assembly of claim1, wherein an output pressure from the multi-stage centrifugalcompressor is between about 2 bara and about 165 bara.
 11. Acompressor-expander assembly, comprising: a cryogenic expanderpositioned in an overhung configuration on a central shaft; amulti-stage centrifugal compressor supported on the central shaftbetween at least two bearings; and an electric motor generatorcombination coupled to the central shaft and configured to supplyrotational power to the central shaft or generate power from rotation ofthe central shaft, depending upon a current operational mode of themulti-stage compressor, wherein the electric motor generator combinationis configured to operate in three modes comprising: a first mode inwhich the cryogenic expander supplies rotational power to the centralshaft and the electric motor generator combination operates to generateelectrical power from the rotation of the central shaft; a second modein which the cryogenic expander supplies rotational power to the centralshaft and the electric motor generator combination supplies additionalrotational power to the central shaft; and a third mode in which thecryogenic expander supplies rotational power to the central shaft andthe electric motor generator combination rotates with the central shaftwithout adding rotational power thereto or generating electric powertherefrom.
 12. The compressor-expander assembly of claim 1, whereinfluid may initially enter the centrifugal compressor assembly radiallyand compressed fluid may exit radially from the centrifugal compressorassembly.
 13. The compressor-expander assembly of claim 1, wherein themulti-stage centrifugal compressor is in a back to back configuration, adouble flow configuration, or a compound compressor configuration.
 14. Amethod comprising: expanding a fluid in a cryogenic expander to rotatethe cryogenic expander and create a rotational output on a central shaftto which a multi-stage centrifugal compressor and a driver/generatordevice are directly coupled for concomitant rotation; driving thecompressor with driver/generator device and the cryogenic expander ifthe output from the cryogenic expander is less than that required todrive the compressor; driving the driver/generator device and thecompressor with the cryogenic expander if the output from the cryogenicexpander is more than that required to drive the compressor; and drivingonly the compressor with the cryogenic expander if the output from thecryogenic expander is not more or less than that required to drive thecompressor.
 15. The method of claim 14, further comprising positioningthe cryogenic expander in an overhung configuration on the centralshaft.
 16. The method of claim 14, further comprising supporting thedriver/generator device on the central shaft.
 17. An apparatus,comprising: an overhung cryogenic expander which receives and cools afluid while generating rotational work; a multi-wheel centrifugalcompressor; a shaft operably coupling the cryogenic expander to receivethe rotational work and also coupled to the multi-wheel centrifugalcompressor, the shaft having a first end portion, a second end portion,and a longitudinal portion disposed between the expander and thecompressor; a bearing rotationally supporting the longitudinal portionof the shaft; a casing enclosing the expander, the multi-wheelcentrifugal compressor, the first end portion of the shaft and thelongitudinal portion of the shaft, with the second end portion of theshaft extending outwardly through the casing and adapted to beoperatively coupled to an external piece of rotating machinery; abalance piston positioned along the shaft between an outlet of themulti-wheel centrifugal compressor the bearing; and a thrust bearingpositioned along the shaft.
 18. The apparatus of claim 17, wherein theinterior of the casing is pressurized.
 19. The apparatus of claim 17,wherein each of the expander and the multi-wheel centrifugal compressorhas an inlet and an outlet, neither of which is directly exposed to theexterior of the casing.
 20. The apparatus of claim 17, wherein theexpander is configured to an input fluid stream that is at a temperatureof between about 50° C. and about −150° C.
 21. The apparatus of claim17, wherein an output pressure from the multi-wheel centrifugalcompressor is between about 2 bara about 165 bara.
 22. Acompressor-expander assembly, comprising: a cryogenic expanderpositioned in an overhung configuration on a central shaft; and amulti-stage centrifugal compressor supported on the central shaftbetween at least two bearings, wherein the compressor-expander mayfurther include: an electric motor generator combination coupled to thecentral shaft and configured to supply rotational power to the centralshaft or generate power from rotation of the central shaft, dependingupon a current operational mode of the multi-stage compressor; and/or arotating machinery device coupled to the central shaft, wherein therotating machinery device is configured to either provide rotationalpower to or receive rotational power from the central shaft; and/orwherein the cryogenic expander is a radial input axial output expander;and/or wherein the bearings comprise at least one of radial magneticbearings and lubricated oil bearings; and/or wherein the cryogenicexpander and the multi-stage centrifugal compressor are contained in asingle casing, and the central shaft extends from the single casing andis coupled to the electric motor generator combination; and/or whereinthe cryogenic expander is contained in a first casing and themulti-stage centrifugal compressor is contained in a second casing, thefirst and second casings being coupled together; and/or wherein thecompressor-expander assembly further comprises a balance pistonpositioned along the central shaft between an outlet of the multistagecentrifugal compressor and one of the at least two bearings, and athrust bearing positioned along the central shaft; and/or wherein thecryogenic expander is configured to receive an input fluid stream thatis at a temperature of between about 50° C. and about −150° C.; and/orwherein an output pressure from the multi-stage centrifugal compressoris between about 2 bara and about 165 bara; and/or wherein the electricmotor generator combination is configured to operate in three modescomprising a first mode in which the cryogenic expander suppliesrotational power to the central shaft and the electric motor generatorcombination operates to generate electrical power from the rotation ofthe central shaft, a second mode in which the cryogenic expandersupplies rotational power to the central shaft and the electric motorgenerator combination supplies additional rotational power to thecentral shaft, and a third mode in which the cryogenic expander suppliesrotational power to the central shaft and the electric motor generatorcombination rotates with the central shaft without adding rotationalpower thereto or generating electric power therefrom; and/or whereinfluid may initially enter the centrifugal compressor assembly radiallyand compressed fluid may exit radially from the centrifugal compressorassembly; and/or wherein the multi-stage centrifugal compressor is in aback to back configuration, a double flow configuration, or a compoundcompressor configuration.
 23. An apparatus, comprising: an overhungcryogenic expander which receives and cools a fluid while generatingrotational work; a multi-wheel centrifugal compressor; a shaft operablycoupling the cryogenic expander to receive the rotational work and alsocoupled to the multi-wheel centrifugal compressor, the shaft having afirst end portion, a second end portion, and a longitudinal portiondisposed between the expander and the compressor; a bearing rotationallysupporting the longitudinal portion of the shaft; a casing enclosing theexpander, the multi-wheel centrifugal compressor, the first end portionof the shaft and the longitudinal portion of the shaft, with the secondend portion of the shaft extending outwardly through the casing andadapted to be operatively coupled to an external piece of rotatingmachinery; a balance piston positioned along the shaft between an outletof the multi-wheel centrifugal compressor the bearing; and a thrustbearing positioned along the shaft; and/or wherein the interior of thecasing is pressurized; and/or wherein each of the expander and themulti-wheel centrifugal compressor has an inlet and an outlet, neitherof which is directly exposed to the exterior of the casing; and/orwherein the expander is configured to an input fluid stream that is at atemperature of between about 50° C. and about −150° C.; and/or whereinan output pressure from the multi-wheel centrifugal compressor isbetween about 2 bara about 165 bara.